This question involves several reactions of copper and its compounds. — Descriptive Chemistry / Stoichiometry Chemistry Question
Problem Context
This question involves several reactions of copper and its compounds.
A sample of copper metal is dissolved in 6 M nitric acid contained in a round bottom flask. This reaction yields a blue solution and emits a colorless gas which is found to be nitric oxide. Write a balanced equation for this reaction.
Model Answer
3Cu + 8H+ + 2NO3– → 3Cu2+ + 2NO + 4H2O
The water is evaporated from the blue solution to leave a blue solid. When the blue solid is heated further, a second reaction occurs. This reaction produces a mixture of nitrogen dioxide gas, oxygen gas and a black oxide of copper.
i. A sample of the dried gas, collected in a 125 mL flask at 35 ˚C and 725 mm Hg, weighed 0.205 g. Find the average molar mass of the gas and the molar NO2/O2 ratio in it.
Model Answer
For the ratio, let x be the fraction of NO2,
46x + 32(1- x) = 43.5
solving for x : x = 0.821
ii. These data were obtained for the black solid;
Mass of empty flask: 39.49 g
Mass of flask + copper metal: 40.86 g
Mass of flask + oxide of copper: 41.21 g
Determine the formula of the oxide of copper.
Model Answer
40.86 g - 39.49 g = 1.37 g Cu
41.21 g - 40.86 g = 0.35 g O
1.36 g Cu × (1 mol Cu / 63.55 g Cu) = 0.0216 mole Cu
0.35 g O × (1 mol O / 16.0 g O) = 0.0219 mole O
Therefore the ratio is 1 : 1 and the formula must be CuO.
If some of the blue solution were lost due to splattering during the evaporation, what would be the effect on the calculated percentage of copper in the black oxide? Explain.
Model Answer
The lost solution will cause the mass of CuO to be too low relative to the mass of Cu. Therefore the percentage determination for copper will be too high
If all of the blue solid were not decomposed into the black oxide during the final heating, what would be the effect on the calculated percentage of copper in the oxide? Explain.
Model Answer
The mass of CuO will be too high. Therefore the percentage determination for copper will be too low.